1. Field of the Invention
The present invention relates to metal seals, and specifically, metal seals used for high temperature pneumatic ducting joints.
2. Description of Related Art
Metal seals for weld-deformed high temperature pneumatic ducting joints are employed to stem leakage in high temperature aeronautical applications, where pressures vary over wide ranges. They are used in many locations where pressures may be high, such as within engine compressor bleed air ducting upstream of a pressure-reducing valve, or where pressures are much lower, downstream of such a device. The bleed pressure also varies in the flight cycle from taxiing to take-off and climb, from high speed cruise to descent, and flight-idle while waiting for a landing slot. Seal leakage in the moderate-to-lower pressure ranges can be a serious concern. Even if direct jet-leakage does not damage non-metallic components, high “under-nacelle temperatures” can cause their premature failure and possibly weaken highly loaded structures. In a jet engine, the nacelle region is generally considered to be composed of the podded engine inlet, fan cowl, thrust reverser, and the exhaust nozzle.
Initial E-shaped seals were described in U.S. Pat. No. 3,192,690, issued to Taylor on Jul. 6, 1965, entitled “SEALING RING WITH E-SHAPED RADIAL SECTION,” and U.S. Pat. No. 3,575,432, issued to Taylor in Apr. 20, 1971, entitled “SEALING RING.” The former was originally intended for high-pressure hydraulic applications and the latter for sealing pneumatic ducting joints. During qualification testing for an aerospace application, the pneumatic E-Seal was subsequently discovered to be prone to fatigue failure in cyclic pressure and bending moment testing of pneumatic ducting joints.
U.S. Pat. No. 3,797,836 issued to Halling on Mar. 19, 1974, entitled “SEALING RING,” teaches an alternative approach where the outer arms of the E-seal have a sinuous configuration. FIG. 1 depicts a pneumatic E-seal of this type.
Ducting manufacturers have continually struggled with maintaining flange face flatness tolerances within the limits defined to meet specified leakage requirements. Multi-ply sealing rings were introduced in an attempt to resolve this problem. A two-ply E-seal was introduced as depicted in FIG. 2 having greater axial height with its free edges sealed together by edge-welding, which permitted larger flatness tolerances.
In U.S. Pat. No. 5,249,814, issued to Halling on Oct. 5, 1993, entitled, “MULTI-PLY SEALING RINGS AND METHODS FOR MANUFACTURING SAME,” and in a related divisional patent, U.S. Pat. No. 5,433,370, issued to Halling on Jul. 18, 1995, under the same title, multi-ply sealing rings are taught having two annular members nested one within the other and welded at equidistant intervals to form a plurality of annular weld zones.
The two-ply edge-welded seal, however, was appreciably expensive to produce. A folded-edge two-ply seal was taught in U.S. Pat. Nos. 5,630,593 and 5,716,052, both entitled, “PRESSURE-ENERGIZED SEALING RINGS,” and issued to Swensen, et al., on May 20, 1997 and Feb. 10, 1998, respectively. This type of seal is currently sold under the trade name U-PLEX, and is depicted in FIG. 3.
Experience has shown that the two-ply seal has drawbacks that make it unsuitable for its intended application. For instance, in order to avoid seal overstress during installation in the standard deep flange cavity, which is approximately 0.088±0.002 inches, the free-height must be restricted to 0.121±0.003 inches. In contrast, the governing specification for weld-deformed flange surface flatness requires that the seal be able to satisfy leakage limits when the local cavity depth increases to a maximum of 0.126 inches over one (1) inch of circumference.
The seal free-height of the U-PLEX seal expands under pressure-energization to close the gap between itself and the flanges, and consequently form a tight seal. This is well-known, and observed for all resilient metallic seals in pneumatic systems, which eventually cut off or reduce leakage when the energizing pressure differential is sufficiently high enough. Until that threshold is reached, however, leakage from the joint is significant. FIG. 6 is a non-deformed ducting joint with the U-PLEX seal of FIG. 3. In contrast, FIG. 7 is a maximum deformed ducting joint with the same U-PLEX seal. In FIG. 6, there are three points of contact shown with the flange surfaces, one at the top or heel and two on the outside legs. The upper leg contact point is removed upon deformation, as depicted in FIG. 7, and only the lower contact sealing point remains as indicated by the arrows. As shown, deformation compromises the integrity of the seal joint.
Given the industry's tendency towards the multi-ply designs, it did not seem probable that a high quality, cost effective sealing ring produced from a single thickness of material was feasible. For example, in U.S. Pat. No. 6,299,178, issued to Halling on Oct. 9, 2001, entitled, “RESILIENT SEALS WITH INFLECTION REGIONS AND/OR PLY DEFORMATIONS,” a single-ply seal with contacting inflexion (internal fulcrum) points had been successfully qualified but its manufacture and implementation was not economically viable.
Moreover, a disadvantage of the prior art designs is the stiffness of the folded back ends (Ref.: FIG. 3), which significantly detracts from the ability of the seal to conform to local depressions in flange sealing surfaces of smaller diameter flanges.